Operation of Wideband Code Division Multiple Access (WCDMA)/High Speed Packet Access (HSPA) on multiple 5 MHz frequency blocks, sometimes referred to as “carriers” is one further step of evolving WCDMA and HSPA. This mode of operation is often referred to as Multi Carrier WCDMA or Multi Carrier HSPA, in the following referred to as “multi carrier HSPA”.
A multi carrier (MC) arrangement with frequency division duplex (FDD) can be described as a set of downlink carriers linked to a set of uplink carriers. The downlink carriers can be adjacent or non-adjacent in the frequency domain, and the same holds for the uplink carriers. More general, carriers do not need to be in the same band and Time division duplex (TDD) bands could also be used as part of the multi carrier operation.
Some of the Radio Frequency (RF) requirements such as receiver sensitivity level or blocking characteristics are generally dependent upon the frequency band of operation as is further described in third Generation Partnership (3GPP) specification No. TS 25.101, “User Equipment (UE) radio transmission and reception (FDD)”, and 3GPP TS 25.104, “Base station (BS) radio transmission and reception (FDD)”.
For instance in WCDMA the User Equipment (UE) receiver sensitivity level for band I (2 GHz) is 3 dB lower than that for band VIII (900 MHz). The differences in RF requirements are not only due to the frequency of operation but other factors such as duplex gap, co-existence scenarios, etc. Thus, multiple carriers from different bands at least for certain bands would exhibit different performance leading to different coverage.
Another aspect of the multi carrier arrangement is the possibility of specifying asymmetrical number of carries in different directions. For instance the number of downlink carriers can be set different from the number of uplink carriers e.g. 4 downlink and 2 uplink carriers. Furthermore, dynamically or semi-dynamically the number of carriers activated at a time can be configured by the network node.
The availability of more than one carrier either in the downlink or uplink or in both directions enables the use of more than one carrier for data transmission. The manner in which multiple carriers can be used for data transmission leads to the following possible transmission modes, which are elaborated further below:                Transmission mode #1: different data blocks over different carriers        Transmission mode #2: interleaved data block over different carriers        Transmission mode #3: repetitive data block over different carriers        Transmission mode #4: repetitive data block over different carrier in different Transmission Time Intervals (TTI)Transmission Mode #1: Different Data Blocks Over Different Carriers        
The main objective of this mode is to increase data rate since the same UE simultaneously receives more than one data block. In case of N activated carriers, the data rate can be increased N times compared to that in single carrier case.
This scheme is used or known in the legacy multicarrier systems such as HSPA, see 3GPP specification No. TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD)”, and advanced Evolved Universal Mobile Telecommunications System Terrestrial Radio Access Network (E-UTRAN) in the downlink and also envisaged for the uplink in advanced E-UTRAN and also in the next phase of HSPA evolution.
However, the UE may not require the use of all possible carriers for downlink or uplink transmissions. Thus, in existing systems the network signals UE to activate or deactivate the use of multiple carriers for transmission #1 in order to save UE power consumption and to reduce the UE and network complexity, see 3GPP specification No. TS 25.214, “Physical layer procedures (FDD)”. Separate signaling is used for uplink and downlink.
Transmission Mode #2: Interleaved Data Block Over Different Carriers
In a multi carrier HSPA system the multiple carriers are typically used for increasing the transmission rate by transmitting different data blocks over different carriers for a same UE. However, in this transmission scheme one data block is sent over more than one carrier. This also enables the data block to benefit from the frequency diversity. However, no combining is required at the receiver node. Instead the decoding of the data block requires the receiver node to demodulate all carriers over which the date block is interleaved. This scheme is envisaged for use in the advanced E-UTRAN. Although this scheme can improve coverage, the main reason is to use this scheme for transmitting a large data block, which is difficult to transmit successfully over a single carrier.
Transmission Mode #3: Repetitive Data Block Over Different Carriers
According to this scheme the coverage can be increased and/or the base station transmission power can be reduced by transmitting the same data block over different carriers to the same UE during the same transmission time interval (TTI) in the downlink. This also leads to frequency diversity. Furthermore this scheme can also be used in the uplink to improve the uplink coverage and/or the UE transmission power.
This scheme may be used for median or low rate services in order to increase the coverage, for instance, low rate services like Voice over Internet Protocol (VoIP) even if one carrier would be sufficient. In this scheme the same data block is repeated over multiple carriers. Thus, in the downlink the UE has to combine the data block received via different carriers. Similarly in the uplink the base station will do the combining if this scheme is used. Compared to scheme #2, this scheme is expected to be less complex for both UE and the Base Station. However from performance perspective the differences may be minor.
The different possible combining schemes will now be described.
Combining Schemes for Transmission Mode #3:
The UE can combine the data block using any of the below methods:                Selective combining        Soft combining        
In selective combining the UE chooses the correctly received data block after separately decoding each block and sends it to the higher layers for further processing.
In soft combining the UE decodes the data block after combining the soft bits or symbols from the data blocks received on different carriers and sends the decoded block to the higher layers.
In both cases the UE sends single acknowledge/negative acknowledge (ACK/NACK) regardless of the carriers used for the transmission of the data block.
Transmission Mode #4: Repetitive Data Block Over Different Carriers in Different TTI
In this scheme the same data block is sent on different carriers but at different TTIs, which is similar to the concept of frequency hopping. The purpose of this scheme is also to improve coverage by the virtue of frequency and time diversity.
There is a constant demand to increase the efficiency of a radio system and to utilize existing resources in an optimal way.